1. Field of the Invention
The present invention relates to a carcass structure for vehicle wheel tyres of the type comprising at least one carcass ply comprising thread-like elements substantially disposed transversely of a circumferential extension of the tyre; at least one pair of annular reinforcing structures disposed close to respective inner circumferential edges, each of said annular reinforcing structures of the carcass structure comprising: at least one first circumferentially-inextensible annular insert substantially in the form of a crown disposed substantially coaxially of the tyre and close to an inner circumferential edge of the carcass ply, said first annular insert being formed of at least one elongated element extending in concentric coils; at least one second circumferentially-inextensible annular insert substantially in the form of a crown disposed coaxially of the tyre, said second annular insert being formed of at least one elongated element extending in concentric coils.
The invention also relates to a method of manufacturing said carcass structure, said method being of the type comprising the steps of: making a carcass ply having a pair of inner circumferential end flaps; applying at least one annular reinforcing structure close to each end flap of the carcass ply.
2. Description of the Related Art
Manufacturing of tyres for vehicle wheels involves formation of a carcass structure essentially consisting of one or more carcass plies substantially having a toroidal conformation and the axially opposite side edges of which being turned back around respective annular reinforcing structures, each of which usually comprises a circumferentially-inextensible metallic annular insert, commonly referred to as “bead core” and a filling of elastomer material coupled with the bead core at a radially outer position.
Applied to the carcass structure, at a circumferentially external position thereof, is a belt structure comprising one ore more belt strips in the form of a closed ring, essentially consisting of textile or metal cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
A tread band currently consisting of a strip of elastomer material of appropriate thickness is applied to the belt structure, at a circumferentially external position thereof.
It is to point out that, to the aims of the present description, by the term “elastomer material” it is intended a rubber blend in its entirety, that is the assembly made up of a base polymer suitably amalgamated with reinforcing fillers and/or additives of other types.
Finally, to the opposite sides of the tyre being manufactured a pair of sidewalls are applied, each of them covering a side portion of the tyre included between a so-called shoulder region, located close to the corresponding side edge of the tread band, and a so-called bead located at the corresponding bead core.
In accordance with traditional production methods, essentially the above listed tyre components are first made separately from each other to be then assembled during a tyre-manufacturing step.
Production methods have been recently proposed which, instead of resorting to the production of semifinished products, make the carcass structure directly during the tyre-manufacturing step.
For example, U.S. Pat. No. 5,362,343 discloses a method and an apparatus forming a carcass ply starting from a single cord previously wound around a reel that at every operating cycle of the apparatus is cut to size for obtaining a section of preestablished length.
The cord section is transversely laid down on the outer surface of a rigid toroidal support previously coated with a layer of raw rubber, then the section ends are radially applied to the respectively opposite sides of the toroidal support itself.
Repetition of the above described operating cycle leads to deposition of a number of cord sections circumferentially disposed in side by side relationship until the whole circumferential extension of the toroidal support has been covered, so as to define a carcass ply.
For the purpose of manufacturing the annular reinforcing structures, it is also known that close to each of the tyre beads, the opposite ends of the individual cords forming a carcass ply are arranged, in an alternating sequence, at axially opposite positions relative to an annular anchoring element made up of coils of metal wire such disposed as to form concentric circumferences arranged substantially in form of a crown, as can be learnt from patent EP 0 664 231.
In the European patent application No. 97830731.2 in the name of the same Applicant, a method and a tyre are described in which each carcass ply is formed by laying down a plurality of strip-like sections following each other onto a rigid toroidal support, each of said sections comprising a plurality of parallel longitudinal cords incorporated into a layer of elastomer material.
When deposition has been completed, each section substantially extends in a U-shaped conformation around the cross-section outline of the toroidal support, so as to exhibit a crown portion extending transversely of the circumferential extension of the toroidal support itself, according to a given angle, and two side portions extending in the direction of the geometric axis of the toroidal support and partly overlapping the side portion of the previously laid-down section.
After the first carcass ply has been manufactured, the annular reinforcing structures are applied at a radially inner position against the end flaps of the ply itself, each of said reinforcing structures comprising a filling insert of elastomer material of a substantially triangular section, interposed between first and second circumferentially-inextensible annular inserts, each formed of a thread-like element wound in concentric coils substantially in the form of a crown.
Then a second carcass ply is formed and disposed upon the first carcass ply and the annular reinforcing structures, said second carcass ply being obtained by strip-like sections laid down in a crossed orientation with respect to those of the first carcass ply.
In the above mentioned patent application it is highlighted that the crossed orientation of the strip-like sections belonging to the first and second carcass plies respectively, as well as the mutually crossed arrangement of the overlapping regions between the sections belonging to the first and second carcass plies respectively, give the bead and sidewall areas of the tyre excellent features in terms of structural resistance in relation to the effects due to slip thrusts and to the twisting moments transmitted in acceleration and braking.